It is known that a variety of structures such as skyscrapers, power-transmission towers, bridges, rapid transit railroads, speedways, golf practice range facility, etc. as well as aircraft may shield and reflect TV, radio and other waves to cause ghosts in an expansive area.
For the purpose of preventing such ghosts, it was proposed to use wave absorbers molded from a concrete, e.g. orthodox concrete, mortar, lightweight concrete, etc., containing carbon fibers as described inter alia, in Japanese Unexamined Patent Publication Nos. 108603/1983 and 108602/1983.
However, the wave-absorbing efficiency of the conventional wave absorber can be improved only by increasing the proportion of carbon fiber or increasing the thickness of the absorber.
The complex relative dielectric constant .epsilon. of a dielectric material is given by: EQU .epsilon.=.epsilon.'-j.epsilon."
wherein the real part .epsilon.' represents a dielectric constant and the imaginary part .epsilon." represents a loss factor.
It is also known that when the real part .epsilon.' and imaginary part .epsilon." values are plotted on the abscissa and the ordinate, respectively, a characteristic curve of D(=d/.lambda..sub.0), where d is the thickness of a sample and .lambda..sub.0 is the wavelength of the electromagnetic wave incident on the sample, can be constructed in the absence of reflection as shown in FIG. 3. The figures given in FIG. 3 are values of D.
By way of illustration, assuming that the waves of TV Channel 2 in Japan (allocated frequency: 96-102 MHz, video carrier frequency 97.25 MHz, audio carrier frequency 101.75 MHz) are absorbed without reflection by a 10 cm-thick wall, the wavelength .lambda..sub.0 of the longer wavelength video carrier wave is: EQU .lambda..sub.0 =(3.times.1010).div.(97.25.times.106).apprxeq.308.48 cm.
Accordingly the value of D is: EQU D=10.div.308.48=0.032
The values of the real part .epsilon.' and imaginary part .epsilon." which satisfy the above value of D is: EQU .epsilon.'.apprxeq.61 EQU .epsilon.".apprxeq.10.
When the thickness of the sample is 5 cm, D=0.016.
It is clear that a larger value is necessary for the real part .epsilon.'.
However, in the conventional carbon fiber-containing wave absorber, it was difficult to realize such a large value for the real part .epsilon.'. Thus, the conventional wave absorber was deficient in wave absorbing capacity.
The present invention has been accomplised under the circumstances described above. The object of the present invention is to provide a wave-absorbing method in which electromagnetic waves can be absorbed with high efficiency even using a carbon fiber-containing composition of reduced carbon fiber content and reduced thickness.